With the advance of semiconductor production technology in recent years, semiconductors are to be produced with a higher and higher packing density and a smaller and smaller line width. Ultrapure water for use as a cleaning liquid in semiconductor production processes is therefore required to have contents of impurities such as trace metals and metal ions being reduced to the limit. Accordingly, strict demands are made on the techniques for removing impurities from ultrapure water to be used in the semiconductor industry.
Ultrapure water, highly purified by an ultrapure water production system, is subjected to further removal of impurities immediately before a point of use (POU) in semiconductor production processes. Filtration is the only one technique as a technique for removing impurities immediately before the point of use.
However, in an ultrapure water production system, complicated operations such as oxidization with ultraviolet rays, ion exchanging, and ultrafiltration are conducted. If complicated operations are further conducted at the point of use, therefore, the system should have an increased size. This causes problems or troubles such that an installation space for the system at the point of use is hardly allocated, a large power is required, and the system itself causes impurities such as metals and metal ions. Accordingly, a strong demand has been made to construct an apparatus and method for filtration, which require a small place and which are capable of further purifying ultrapure water to a target purity.
The most general process for removing trace metals and metal ions in water is a process of carrying out ion exchanging of metals and metal ions with a bead-like ion exchange resin or chelating resin to remove the metals and metal ions from the water.
When metals and metal ions are removed by using such a bead-like ion exchange resin, however, the metal ions are removed only at a low rate, because a speed at which the metals and metal ions diffuse into the ion exchange resin limits this rate. For providing such an ion exchange efficiency as to be usable in actual process, therefore, an apparatus for use herein should have a large size to increase the capacity of the ion exchange resin. Thus, size reduction of the filtration apparatus is difficult.
As a possible solution to these problems, there are proposed filters each including a porous membrane, such as a porous flat membrane or hollow-fiber membrane, or a nonwoven fabric as a base material, in which the base material bears a functional group such as an ion exchanging group or chelating group (Japanese Unexamined Patent Application Publication No. 11-99307 and Japanese Unexamined Patent Application Publication No. 2005-218947).
A filter of this type acts to remove metal ions, because the functional group has an ability of removing metal ions. Additionally, the filter achieves a high rate of removing adsorbed molecules, because the metal ions are transported into pores of the filter by convection of ultrapure water.
However, this filter, if using a hollow-fiber membrane as the base material, causes increased cost.
The filter, if using a flat membrane as the base material, undergoes breakthrough within a short time, because the membrane thickness thereof is small, although the filter can remove metals and metal ions at a high removal rate in early stages of water passing. For avoiding this problem, the membrane is to have a larger size so as to reduce the amount of metal ions to be removed per unit membrane area. This, however, makes the filtration apparatus have a larger size. For avoiding the larger-size apparatus, two or more plies of membranes may be used. This, however, increases pressure loss.
Japanese Examined Patent Application Publication No. 3-9798 describes a treatment typically of city water with a macroreticular ion exchange resin and an ion exchange fiber.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-99307
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-218947
Patent Document 3: Japanese Examined Patent Application Publication No. 3-9798
As is described above, demands have been made to further improve the quality of ultrapure water. However, no technique has been found, according to which very trace amounts of metal ions contained in ultrapure water can be stably removed to a high degree within a limited space, such as one immediately before a point of use, over a prolonged period of time.